This invention relates to a method for purifying a dianhydride, the dianhydrides formed thereby, and polyetherimides formed therefrom. More particularly, the method relates to purifying an oxydiphthalic anhydride.
Oxydiphthalic anhydride, a monomer prized as a component of a unique class of high temperature polyetherimides, may be prepared from chlorophthalic anhydride by coupling two molecules of chlorophthalic anhydride in the presence of an inorganic carbonate, a solvent and a phase transfer catalyst. The crude product of such a coupling reaction often includes the solvent, unreacted starting material(s), phase transfer catalyst, inorganic by-products, and other impurities, which must be separated from the oxydiphthalic anhydride prior to its use in polymer synthesis.
The purification of anhydrides generally has been the focus of an extensive research effort. For example, various processes have been advanced for the purification of phthalic anhydride and pyromellitic acid using activated carbon and are disclosed in U.S. Pat. Nos. 1,301,388; 2,937,189; 2,985,665; 3,236,885; and 3,236,885. U.S. Pat. No. 2,786,805 teaches that phthalic anhydride can be purified by making a slurry of the material in water, heating the slurry to 375° F. to 400° F. (191° C. to 204° C.), removing the anhydride by passing steam into the mixture and condensing the purified phthalic anhydride vapors. U.S. Pat. No. 3,338,923 discloses a method of purifying pyromellitic dianhydride by treatment with ketones. Furthermore, U.S. Pat. No. 3,338,923 discloses that the material can be purified by converting the dianhydride into the corresponding acid with water and recrystallizing the acid from water in the presence of activated carbon.
U.S. Pat. No. 4,906,760 discloses the removal of various metal ion impurities from aromatic anhydrides. U.S. Pat. No. 4,906,760 likewise discloses the removal of metal ion impurities from aromatic anhydrides.
U.S. Pat. No. 4,870,194 discloses a purification scheme for oxydiphthalic anhydride. U.S. Pat. No. 5,145,971 likewise discloses a process for the preparation of purified oxydiphthalic acid from impure oxydiphthalic anhydride. U.S. Pat. No. 5,336,788 discloses the conversion of oxydiphthalic acid to oxydiphthalic anhydride
Previous research efforts and achievements notwithstanding, there is a continuing need to develop improved processes for the purification of oxydiphthalic anhydrides. Purification of oxydiphthalic anhydrides is challenging, in part, because certain materials used in methods are soluble in organic solvents while other components are substantially insoluble in the organic solvents. The disparate solubility properties of the materials make it difficult to create a single process that addresses such different solubility properties. The disparate solubility properties of the materials also make it difficult to obtain a high purity/high quality oxydiphthalic anhydride. Further, purification of oxydiphthalic anhydrides is challenging, in part, because manufacturing conditions require processes to produce high purity/high quality oxydipthalic anhydrides economically with minimal use of solvent.
For the foregoing reasons, there is a need to develop methods for purifying dianhydrides, and in particular oxydiphthalic anhydrides, that consume the least solvent without degrading yield or purity.